The invention relates to a device for removing the gaseous laminar boundary layer from at least one of the two sides of a material web, paper for example, said web being moved in the transport direction, preferably at high speed.
Gaseous laminar boundary layers on material webs moved in air are known of themselves to be troublesome. Thus, for example when material webs are wound on a roll to form paper or film bales, including the laminar boundary layer in the winding produces a larger bale diameter for the actual length of material web to be wound than when the boundary layer is not included. In addition, for example during drying in a press, an attempt is made to expel the solvent in the printing ink(s) from the material web by drying. So-called boundary layer doctors are known for this purpose, with air jets mounted at right angles or transversely to the surface, said jets blowing air at high energy and speed against the material web to convert the laminar boundary layer in the microscopic range into a turbulent flow whose vortices have increasingly larger diameters than the thickness of the boundary layer, so that firstly they allow the solvent from the printing ink to pass through better and secondly they can be influenced by conventional blowing and/or suction nozzles for macroscopic elimination.
Such systems for drying are known especially for example in gravure, rotary-offset, and flexographic printing. In all of these printing methods, the ink dries as a result of the expulsion of the solvent or solvents, which are hydrocarbons or mixtures of alcohol and water. Because of the high speed at which the material web is transported, pronounced laminar boundary layers result that hinder both the transport of heat into the material web and the transport of solvent material out of it. Both physical principles are significant for drying.
Heat transport in a dryer based on hot air systems is responsible for the heating and therefore for the temperature increase of the material web. As a result of heat transport, the energy required to expel the solvent is provided. On the other hand, the material transport corresponds to its solvents driven out of the material web. Since drying is performed as a rule with material web temperatures above 100.degree. C., there is still a small amount of water present that evaporates from the paper.
It is clear that the quality of a drying system depends upon the highest possible transfer of heat and material with a simultaneous small temperature differential between the ambient air and the material web. A small temperature differential necessarily means that there is a lower energy requirement provided the rest of the dryer system remains the same.
It is known from flow technology that in particular laminar boundary layers with relatively low Reynolds numbers, in conjunction with the high kinematic viscosity of hot air, exhibit low thermal and material transfer.
Since heat and material transfer in turbulent flow is a multiple of the value for laminar flow, an attempt is made in known drying systems to convert the laminar boundary layer both pneumatically and mechanically to change it to a turbulent boundary layer using the above-mentioned boundary layer doctor. As a rule, specially designed blowing nozzles are used, directed against at least one side of the material web. The results are not satisfactory despite the application of high energy. The reason is not known exactly despite intensive research. Possibly the reason for the unsatisfactory results lies in the fact that with surface roughnesses of the material web of approximately 2 to 4 .mu.m in the case of paper, despite the conversion from a laminar to a turbulent boundary layer, a thin laminar so-called residual boundary layer is embedded so to speak in the unevennesses in the surfaces of the material web produced by the roughness, so that heat and material transport are both hindered.
This so-called boundary layer doctor can be used not only in dryers in printing presses, but theoretically in all other applications as well. Nevertheless, the efficiency remains poor.